THE EARTH 115
heat produces further separation between the superficial molecules, so that even the slight cohesive power of liquids is overcome, and the particles assume the vaporous state. Thereupon they are raised into the air, and drifted about by the winds, which themselves result from the separative action of heat. After a time, the particles lose by radiation and convection much of their Kinetic Energy, and begin once more to
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aggregate into rain-drops. These fall upon the surrounding heights, and finally find their way again into the lake. But the Energy which raised them has by this time been dissipated, and fresh Energy will be required to make them once more assume the form of vapour. Nor is this all. As the drops fall upon the mountain, side, they employ part of their Energy in overcoming the cohesion of its molecules. In this way they slowly wear away the elevations on the earth's surface, and carry clown their particles to the valleys or the sea. In so doing, they act as liberating agents for the Potential Energy of these masses, and so assist in working out the general plan of aggregation. It is true that new mountains are from time to time slowly upheaved in place of the old ones, but these are themselves mere apparent exceptions, as they really represent a general lapse of the surrounding crust: and their heights are in turn worn down by watercourses, glaciers, and percolation. In short,
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